Week 09 Tutorial Questions

Objectives

develop an understanding of how the UNIX file permissions system works
understand and practice coding with UTF-8 encoded characters
introduce environment variables and how programs can access and manipulate them

Explain how this program uses stat to get information about the permissions and type of a file. The inode man page may he helpful here.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>

void print_some_file_info(char *filename);

int main(int argc, char *argv[]) {
        // Loop over command line arguments
        for (int arg = 1; arg < argc; arg++) {
                print_some_file_info(argv[arg]);
        }
}

void print_some_file_info(char *filename) {
        // Use stat to get file information
        struct stat s;

        // check that stat worked
        if (stat(filename, &s) != 0) {
                perror(filename);
                return;
        }

        // get the mode (permissions) of the file
        mode_t mode = s.st_mode;

        // check if the file is a directory
        if ((mode & S_IFDIR) == S_IFDIR) {
                printf("The directory '%s' has mode 0x%x == 0%o\n", filename, mode, mode);

        // check if the file is a regular file
        } else if (S_ISREG(mode)) {
                printf("The file '%s' has mode 0x%x == 0%o\n", filename, mode, mode);
        }

        // check if the file is publically readable
        if (mode & S_IROTH) {
                printf("%s is publically readable\n", filename);
        }
}

Could you add another if statement to check if the file is publically writeable? How would you check if the file can be executed by the owner?

Write a file print_metadata.c, which is given a file path as a command line argument and prints the file size (in bytes) and the file permissions as a string in a format similar to ls -l (e.g. -rw-r--r--). For file type, only directory d and regular file - cases are required to be handled.

Example:
```
echo -n "hello" > demo.txt
chmod 640 demo.txt
ls -l demo.txt
-rw-r-----  1 user  group  5 Sep  4 17:00 demo.txt
dcc print_metadata.c -o print_metadata
./print_metadata demo.txt
size: 5
perms: -rw-r-----
```

Write a C program, chmod_if_public_write.c, which is given 1+ command-line arguments which are the pathnames of files or directories

If the file or directory is publically-writeable, it should change it to be not publically-writeable, leaving other permissions unchanged.

It also should print a line to stdout as in the example below

dcc chmod_if_public_write.c -o chmod_if_public_write
ls -ld file_modes.c file_modes file_sizes.c file_sizes
-rwxr-xrwx 1 z5555555 z5555555 116744 Nov  2 13:00 file_sizes
-rw-r--r-- 1 z5555555 z5555555    604 Nov  2 12:58 file_sizes.c
-rwxr-xr-x 1 z5555555 z5555555 222672 Nov  2 13:00 file_modes
-rw-r--rw- 1 z5555555 z5555555   2934 Nov  2 12:59 file_modes.c
./file_modes file_modes file_modes.c file_sizes file_sizes.c
removing public write from file_sizes
file_sizes.c is not publically writable
file_modes is not publically writable
removing public write from file_modes.c
ls -ld file_modes.c file_modes file_sizes.c file_sizes
-rwxr-xr-x 1 z5555555 z5555555 116744 Nov  2 13:00 file_sizes
-rw-r--r-- 1 z5555555 z5555555    604 Nov  2 12:58 file_sizes.c
-rwxr-xr-x 1 z5555555 z5555555 222672 Nov  2 13:00 file_modes
-rw-r--r-- 1 z5555555 z5555555   2934 Nov  2 12:59 file_modes.c

Make sure you handle errors.

Consider the lseek(fd, offset, whence) function.
1. What is its purpose?
2. When would it be useful?
3. What does its return value represent?
Consider a file of size 10000 bytes, open for reading on file descriptor fd, initially positioned at the start of the file (offset 0). What will be the file position after each of these calls to lseek()? Assume that they are executed in sequence, and one will change the file state that the next one deals with.
1. lseek(fd, 0, SEEK_END);
2. lseek(fd, -1000, SEEK_CUR);
3. lseek(fd, 0, SEEK_SET);
4. lseek(fd, -100, SEEK_SET);
5. lseek(fd, 1000, SEEK_SET);
6. lseek(fd, 1000, SEEK_CUR);
Write a C program, print_file_bits.c, which given as a command line arguments the name of a file contain 32-bit hexadecimal numbers, one per line, prints the low (least significant) bytes of each number as a signed decimal number (-128..127).
What does the following printf(3) statement display?
```
printf ("%c%c%c%c%c%c", 72, 101, 0x6c, 108, 111, 0x0a);
```
Try to work it out without simply compiling and running the code. The ascii(7) manual page will help with this; read it by running man 7 ascii. Then, check your answer by compiling and running.
Write a C program, recurse_sample.c, that walks a directory tree and prints each matching file’s permissions (ls -l style) and pathname when the filename contains the substring "hello".
1. Part 1 — Print CWD: Get and print the current working directory pathname.
  Hint: getcwd()
2. Part 2 — List current directory: Print one line per entry in the current directory (non-recursive):
  <perms> <pathname>
  Hint: use readdir() to recurse through subdirectories
3. Part 3 — Recurse: Recurse into all subdirectories and print pathnames in the same format as Part 1.
4. Part 4 — Filter by name: Only print lines for files whose basename contains "hello" (case-sensitive match).
Output format (one match per line):
-rw-r--r-- ./examples/hello.txt

Example (setup & run):
```
mkdir -p examples/sub
: > examples/hello.txt
: > examples/sub/nothe11o.c
: > examples/sub/hello_world.c
dcc -o recurse_sample recurse_sample.c
./recurse_sample .
-rw-r--r-- ./examples/sub/hello_world.c
-rw-r--r-- ./examples/hello.txt
```
Recall the UTF-8 encoding for characters below:

Why did UTF-8 replace the ASCII coding standard? What is the difference in how ASCII and UTF-8 codepoints are represented in bytes?

Write a C program that reads a null-terminated UTF-8 string as a command line argument and counts how many Unicode characters (code points) it contains. Assume that all codepoints in the string are valid. Some examples of how your program should work:

    dcc count_utf8.c -o count_utf8
    ./count_utf8 "チョコミント、よりもあなた！"
    there are 14 codepoints in the string
    ./count_utf8 "早上好中国现在我有冰淇淋"
    there are 12 codepoints in the string
    ./count_utf8 "🤓🤓🤓🤓🤓🤓🤓🤓"
    there are 8 codepoints in the string

Write a C program, print_diary.c, which prints the contents of the file $HOME/.diary to stdout
The lecture example getstatus.c shows how to get the value of an environment variable.
snprintf is a convenient function for constructing the pathname of the diary file.

Extra questions

The following questions are extra content not necessarily needed to cover this week's content.
Your tutor may still choose to cover some of these questions, time permitting.

The following code will compile with

gcc   void-pointer.c -o void-pointer
clang void-pointer.c -o void-pointer
dcc   void-pointer.c -o void-pointer

It will even compile with

gcc   -Werror -Wall -Wextra void-pointer.c -o void-pointer
clang -Werror -Wall -Wextra void-pointer.c -o void-pointer
dcc   -Werror -Wall -Wextra void-pointer.c -o void-pointer

But it wont compile with

gcc   -Werror -Wall -Wextra              -Wpedantic void-pointer.c -o void-pointer
clang -Werror -Wall -Wextra -Weverything -Wpedantic void-pointer.c -o void-pointer
dcc   -Werror -Wall -Wextra              -Wpedantic void-pointer.c -o void-pointer

How can the code be modified to compile with the -Wpedantic option?

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

static uint16_t a;

int main(void)
{
    uint16_t b;

    uint16_t *pa = &a;
    uint16_t *pb = &b;
    uint16_t *pc = malloc(sizeof(uint16_t));

    *pa = 0xAAAA;
    *pb = 0xBBBB;
    *pc = 0xCCCC;

    printf("a:\n\tvalue: 0x%X\n\taddress: %16p\n", *pa, pa);
    printf("b:\n\tvalue: 0x%X\n\taddress: %16p\n", *pb, pb);
    printf("c:\n\tvalue: 0x%X\n\taddress: %16p\n", *pc, pc);
}

The following code will compile with

gcc   pointer-arithmetic.c -o pointer-arithmetic
clang pointer-arithmetic.c -o pointer-arithmetic
dcc   pointer-arithmetic.c -o pointer-arithmetic

It will even compile with

gcc   -Werror -Wall -Wextra pointer-arithmetic.c -o pointer-arithmetic
clang -Werror -Wall -Wextra pointer-arithmetic.c -o pointer-arithmetic
dcc   -Werror -Wall -Wextra pointer-arithmetic.c -o pointer-arithmetic

But it wont compile with

gcc   -Werror -Wall -Wextra              -Wpedantic pointer-arithmetic.c -o pointer-arithmetic
clang -Werror -Wall -Wextra -Weverything -Wpedantic pointer-arithmetic.c -o pointer-arithmetic
dcc   -Werror -Wall -Wextra              -Wpedantic pointer-arithmetic.c -o pointer-arithmetic

How can the code be modified to compile with the -Wpedantic option?

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

void *next_byte(void *byte){
    return byte + 1;
}

int main(void)
{
    uint8_t  chars[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    uint32_t ints[10]  = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    printf("%16p - %16p\n", (void *)&chars[0], (void *)next_byte(&chars[0]));
    printf("%16p - %16p\n", (void *)&ints[0],  (void *)next_byte(&ints[0]));
}

Revision questions

The following questions are primarily intended for revision, either this week or later in session.
Your tutor may still choose to cover some of these questions, time permitting.